An ideal AC relay switch completes connection or disconnection of its contact at the moment of a zero-crossing point of current. However, due to the time for the movement process of a mechanical contact, a mechanical contact switch cannot be closed or opened at the zero-crossing point theoretically; especially under circumstances of high voltages and high currents, severe sparking and arc discharge phenomenon occur particularly when inductive and capacitive loads are switched on and off, leading to a shortened service life of the switch contact and a potential surge current or voltage harmful to the power grid. Considering that it is impossible for AC relay switches to close or open at the zero-crossing point of current, engineers have attempted to find a way of eliminating the arc discharge at the switch contact from the time when the AC switches were put into use. The emergency of a Silicon Controlled Rectifier (SCR) enables an electronic AC switch which may be closed or opened at a zero-crossing point of current. Nevertheless, due to power consumption and cost problems, the AC switch adopting the SCR cannot be practical and reliable. Meanwhile, numerous researches have performed researches on a low-power-consumption and low-cost composite switch in which an SCR and a relay contact are connected in parallel. However, the SCR is defective for a possible erroneous turning on in the case of high dv/dt and a difficulty in turning off in the case of high di/dt, the SCR composite switch has not been applied practically.
U.S. Pat. Nos. 3,223,888 and 3,284,684 and a Chinese patent application No. 01111050.3 disclose a way of protecting a main relay contact by using a diode, that is, a switch with a mechanical breakpoint upon opening of the switch, where a diode ensures that a contact of the primary switch is applied with merely a forward voltage of the diode at the moment when the contact of the primary switch is connected or disconnected. However, the circuits disclosed in these patents have strict requirements for contact travel time (i.e. duration from the time when the actuation of the contact begins to the time when the actuation of the contact fully stops) of both the secondary relay and the primary relay. For resistive and inductive loads, the switch contact should be connected or disconnected within ½ of an AC cycle; for a capacitive load, it is required that the travel time of the relay contact is less than ¼ of the AC cycle. That is, in the case of 50 Hz AC current, the travel time of the contact is less than 10 mS for the resistive and inductive loads and is less than 5 mS for the capacitive load. In the case of 60 Hz AC current, the desired travel time of the contact is even shorter, and cannot be achieved by general switch relay contacts. Moreover, with the increase of use time of the relay, the contact travel time varies and is prolonged. The existing relays cannot meet the requirements for the switch contact protection by the serial connection between a diode and an auxiliary relay contact as disclosed in the above patents. Therefore, the above patents do not mention which relays are applicable to fulfill the contact protection function, which is also the reason why the circuit of the switch with contact protection based on a diode has not been applied after being proposed more than half a century ago.
With regard to an AC switch relay, jitter sparking takes places at a contact of the AC switch relay when the contact is connected, and arc discharging takes place when the contact is disconnected, while the arc extinguishes at the zero-crossing point of the AC current. The speed of the actuation of the contact is required to ensure that the arc will not be reignited after extinguishing, thus the travel time of the contact of the AC switch relay is required to be short enough. Because jitter happens to the relay contact when the relay contact is connected, damping is generally enhanced in the mechanical system of the relay in order to decrease the number of jitters of the contact. For the purpose of shortening the travel time of the relay contact, a drive current of a relay coil is increased or the damping of the mechanical system is decreased; but the increase of the drive current of the relay coil or the decrease of the damping of the mechanical system makes the jitter upon the connection of the contact more severe, thereby degrading performances of connecting the contact and shortening the mechanical service life of the relay. Thus, it is rather difficult for the AC switch relay to improve its contact actuation speed on the premise that its mechanical service life is guaranteed.